Back pain, particularly in the small of the back, or lumbosacral region (L4-S1) of the spine, is a common ailment. In many cases, the pain severely limits a person's functional ability and quality of life. Back pain interferes with work, routine daily activities, and recreation. It is estimated that Americans spend $50 billion each year on low back pain alone. It is the most common cause of job-related disability and a leading contributor to missed work.
Through disease or injury, the laminae, spinous process, articular processes, facets and/or facet capsule(s) of one or more vertebral bodies along with one or more intervertebral discs can become damaged which can result in a loss of proper alignment or loss of proper articulation of the vertebra. This damage can result in anatomical changes, loss of mobility, and pain or discomfort. For example, the vertebral facet joints can be damaged by traumatic injury or as a result of disease. Diseases damaging the spine and/or facets include osteoarthritis where the cartilage of joints is gradually worn away and the adjacent bone is remodeled, ankylosing spondylolysis (or rheumatoid arthritis) of the spine which can lead to spinal rigidity, and degenerative spondylolisthesis which results in a forward displacement of the lumbar vertebra on the sacrum. Damage to facet joints of the vertebral body often can also results in pressure on nerves, commonly referred to as “pinched” nerves, or nerve compression or impingement. The result is pain, misaligned anatomy, and a corresponding loss of mobility. Pressure on nerves can also occur without facet joint pathology, e.g., a herniated disc.
One conventional treatment of facet joint pathology is spine stabilization, also known as intervertebral stabilization. Intervertebral stabilization desirably controls, prevents or limits relative motion between the vertebrae, through the use of spinal hardware, removal of some or all of the intervertebral disc, fixation of the facet joints, bone graft/osteo-inductive/osteo-conductive material (with or without concurrent insertion of fusion cages) positioned between the vertebral bodies, and/or some combination thereof, resulting in the fixation of (or limiting the motion of) any number of adjacent vertebrae to stabilize and prevent/limit/control relative movement between those treated vertebrae. Stabilization of vertebral bodies can range from the insertion of motion limiting devices (such as intervertebral spacers, artificial ligaments and/or dynamic stabilization devices), through devices promoting arthrodesis (rod and screw systems, cable fixation systems, fusion cages, etc.), up to and including complete removal of some or all of a vertebral body from the spinal column (which may be due to extensive bone damage and/or tumorous growth inside the bone) and insertion of a vertebral body replacement (generally anchored into the adjacent upper and lower vertebral bodies). Various devices are known for fixing the spine and/or sacral bone adjacent the vertebra, as well as attaching devices used for fixation.
One common concern with existing spinal fusion techniques relates to the increased stresses experienced in vertebra adjacent to fused spinal levels. Where one or more functional spine units (a functional spinal unit comprising a pair of adjacent vertebrae and the intervertebral disc and facet joints there between) are fused (or motion is reduced or limited in some manner), the stresses and strains normally accommodated by that flexible unit (now fused or less flexible) are transferred (at least partially) to adjacent spinal units. Where these increased stresses begin to damage and/or degrade other spinal units—which can often occur in levels directly adjacent to the fused level(s)—the degradation is often called “adjacent-level disease” or adjacent segment disease. See, Kulkami, et al. “Accelerated spondylotic changes adjacent to the fused segment following central cervical corpectomy: magnetic resonance imaging study evidence” J. Neurosurg. 100 (1 Suppl Spine):2-6 (2004). Where an adjacent level degrades to the point of requiring surgical intervention, the affected/degraded spinal unit is generally fused (or motion is limited and/or controlled in some manner), further exacerbating the stresses experienced by the remaining unfused levels, and often resulting in multiple-level or “daisy chained” fusions to the spine over time. The spine can be fused using, for example, a spinal fixation system.
More recently, various treatments have been proposed and developed as alternatives to spinal fusion. Many of these treatments seek to restore (and/or maintain) some or all of the natural motion of the treated spinal unit, and can include intervertebral disc replacement, facet joint resurfacing, and facet joint replacement. Such solutions typically include devices that do not substantially impair spinal movement.